Through recent developments in engine technology, in particular through the operating of Otto engines at substantially increased pressures, the stresses to which spark plugs are exposed have increased considerably. Through this, damages occur very frequently to the ceramic of the insulator, in particular in the region of a front, exposed section of the insulator facing the combustion chamber of the engine, hereinbelow also designated as the insulator foot, which in high performance engines is exposed to very high thermal, mechanical, electrical and thermo-mechanical stresses. The occurrence of cracks in the insulator foot is especially problematic. These can be caused by engine vibrations. Uncontrolled combustions can lead to intensive pressure waves or shock waves, which can lead to fractures of the ceramic insulator. The fuel, which is injected at high pressure can, if it comes in contact with the ceramic insulator, trigger a temperature shock, which can likewise lead to cracks in the ceramic. A spark plug of the type named in the introduction is known from DE 10 2011 002 167 A1, in which in view of the above-mentioned problems it is proposed, for improvement, to construct the spark plug so that the insulator foot lies within the spark plug body and is thereby shielded and protected from shock waves and temperature shocks.
Furthermore, in modern high performance engines it is important that the spark plug is able to reliably inflame the frequently very lean fuel-air mixture. One speaks in terms of a lean fuel-air mixture when the ratio of the air quantity actually present in the combustion chamber to the air quantity necessary stoichiometrically for a complete combustion of the fuel is greater than 1. The ignition voltages are therefore increased in high performance engines and can be up to 35 kV. Also in the case of high ignition voltages, the surface region of the insulator which lies at the front end of the insulator between the center electrode and the spark plug body, must form an insulating section, which reliably prevents the occurrence of sliding sparks between the center electrode and the spark plug body. One speaks in terms of a sliding spark when the ignition spark spreads out at least in sections along the surface of the insulator or sweeps over the surface of the insulator. In contrast thereto, one speaks in terms of an air spark when the ignition spark forms over a free air gap between the center- and ground electrode, without touching other components. Sliding sparks should not occur in modern high performance engines, because they can no longer reliably ignite very lean fuel-air mixtures. Sliding sparks can therefore lead to undesired ignition failures. A configuration of the front end of the insulator in the form of an insulator foot has for a long time proved successful in order to reliably prevent the occurrence of sliding sparks. DE 10 2011 002 167 A1 therefore retains the proven concept of a spark plug with an insulator foot and shields the latter from influences from the combustion chamber.
From the documents DE 24 37 257 A1, DE 35 44 176 C1 and DE 38 16 968 A1, which are already over 25 years old, non-generic spark plugs are known, which have combined sliding- and air spark sections. In the spark plugs disclosed there, the front end of the spark plug body also projects over the front end of the insulator. In addition, at the front end of the spark plug body a seal seat, cooperating with the front end of the insulator, is provided. At the front end of the insulator, a recess is arranged, open to the front end of the spark plug body, into which recess the center electrode projects. At the front end of the spark plug body an inwardly projecting collar is provided, on which the seal seat is arranged. The collar at the front end of the spark plug body serves at the same time as a ground electrode. For this, the internal diameter of the collar is smaller than the internal diameter of the recess at the front end of the insulator in the region adjoining the seal seat. Such spark plugs have already no longer been used in practice for a long time because, as already mentioned, a development of a sliding spark is no longer desired.
Furthermore, non-generic spark plugs are known from WO 2009/039478 A2 and DE 11 2008 002 535 T5, in which in fact a recess which is open towards the front end of the spark plug body is arranged at the front end of the insulator, into which recess the center electrode projects and forms with the ground electrode an air spark section, in which, however, the front end of the insulator projects out from the front end of the spark plug body. Hence, the ceramic of the insulator is exposed there in an unprotected manner to the previously described stressing influences. In addition, the seal seat between the insulator and the spark plug body is moved very far toward the back, so that the distance between the seal seat and the front end of the spark plug body is very great. In the region between the seal seat and the front end of the spark plug body, an annular gap is provided between the insulator and the spark plug body, so that the insulator is neither supported nor guided in radial direction in a very long region from the seal seat up to its front end. The insulator is therefore under great risk of fracture by engine vibrations. This configuration has therefore not found its way into practice.